Generally, a thick film resistor (hereinafter also merely referred to as a resistor) is produced by forming a film composed of a resistive composition that contains, as main components, a conductive component and glass on various insulating substrates and firing this. Specifically, the resistive composition, usually in a form of a paste or paint, is printed on an alumina substrate in which electrodes are formed or on a ceramic composite part, and the like to have a predetermined shape and is then fired at a high temperature of 600° C. to 900° C. Thereafter, a protective coating is formed by an overcoat glass if necessary, and then a resistance value is adjusted by laser trimming or the like if necessary.
The characteristics of the resistor to be required are a small temperature coefficient of resistance (TCR), a small current noise, favorable withstand voltage characteristics, favorable process stability (for example, a small change in resistance value by a variation in process), and the like.
Conventionally, a resistive composition using, as a conductive component, ruthenium-based oxide powder (hereinafter also referred to as a ruthenium-based resistive composition) has been generally used widely. This ruthenium-based resistive composition can be fired in air, and by changing the ratio between the conductive component and the glass, resistors having a wide range of resistance value can be easily obtained.
As the conductive component of the ruthenium-based resistive composition, ruthenium dioxide (hereinafter also referred to as ruthenium(IV) oxide); ruthenium composite oxides, such as bismuth ruthenate, lead ruthenate or the like having a pyrochlore structure, barium ruthenate, calcium ruthenate or the like having a perovskite structure; or ruthenium precursors such as ruthenium resinate or the like are used. Especially, in a resistive composition having a high content of glass in a high resistance range, the above-mentioned ruthenium composite oxides such as bismuth ruthenate or the like are preferably used rather than ruthenium dioxide. This is because the resistivity of the ruthenium composite oxides is generally higher by an order of magnitude or more, compared with ruthenium dioxide, and a larger amount of ruthenium composite oxides can be blended compared with ruthenium dioxide, and thus, a variation in resistance value is small, current noise characteristics and resistance characteristics such as TCR and the like are favorable, and stable resistors can be easily obtained.
On the other hand, as the glass used as a component configuring the thick film resistor, mainly a glass containing lead oxide is used. The main reason of this is that the lead oxide-containing glass has a low softening point and has superior characteristics suitable for forming the thick film resistor, such as having a favorable fluidity, wettability with the conductive component, superior adhesiveness to a substrate, and a coefficient of thermal expansion suitable for ceramics, particularly an alumina substrate.
However, the lead component possesses a toxicity and is not desirable from the viewpoint of its effect on the human body and pollution. In order to deal with recent environmental problems, electronics products are required to comply with the Directive of WEEE (Waste Electrical and Electronic Equipment) and RoHS (Restriction of the Use of the Certain Hazardous Substances), and amid this situation, the development of a lead-free material is strongly required for the resistive compositions.
Furthermore, the lead component has very good wettability to alumina. Therefore, the lead component is wet and excessively spread over the alumina substrate at the time of firing, and the shape of the finally obtained resistor becomes an unintended shape in some cases.
Therefore, conventionally, some resistive compositions using, as a conductive component, bismuth ruthenate, alkaline earth metal ruthenate, or the like and using glass containing no lead have been proposed (see PTL (Patent Literature) 1 and PTL 2).
However, a thick film resistor using a lead-free glass and showing superior characteristics comparable with a conventional thick film resistor using lead-containing glass over a wide resistance value range has not been obtained yet. Especially, it has been difficult to form a resistor in a high resistance range of 100 kΩ/□ or more. The reason for this is considered to be as follows.
Many ruthenium composite oxides generally used in a high resistance range are prone to react with a glass to decompose to ruthenium dioxide having a lower resistivity than the ruthenium composite oxide at the time of firing the resistive composition at a high temperature. Especially when the ruthenium composite oxide is used in combination with a glass containing no lead component, it is difficult to suppress the decomposition to ruthenium dioxide at the time of firing (for example, about 800° C. to 900° C.). Therefore, the resistance value is reduced, a desired high resistance value cannot be obtained, and further, there are problems of increasing the dependency on film thickness and the dependency on firing temperature.
By using a ruthenium composite oxide powder having a large particle size (for example, an average particle size of 1 μm or more) as described in PTL 1, the above-mentioned decomposition can be suppressed to a certain extent. However, in the case of using such a coarse conductive powder, the current noise and the load characteristics deteriorate, and favorable resistance characteristics cannot be obtained.
In order to suppress the decomposition of bismuth ruthenate that is one of the ruthenium composite oxides, using a bismuth-based glass as described in PTL 2 in combination is effective. However, the TCR of a resistor obtained by the resistive composition of this combination becomes large in a negative direction in a high resistance range.
A fired film of a resistor was observed with an electron microscope by the inventors of the present invention, and a sign of forming a network (network structure) in which fine conductive particles are dispersed in a matrix of glass and are in contact with one another are observed. Therefore, it is considered that such a network becomes a conductive path, and thus, conductivity is shown.
In known resistive compositions using a combination of ruthenium composite oxide and lead-free glass, it is extremely difficult to stably form the above-mentioned network structure (hereinafter also referred to as a conductive network) especially in a high resistance range in which the content of conductive particles is small. Therefore, a thick film resistor containing no lead and being superior in various characteristics such as TCR characteristics, current noise characteristics, and variations have not become industrially applicable yet.